The impact of fat emulsification on lipid metabolism
Interview with Marie-Caroline MICHALSKI
Marie-Caroline Michalski, INRAE Research Director (h-index=40, ≈ 3200 citations excluding self-citations), studies the impact of the structure and supramolecular organisation of lipids in food on their effects on cardiometabolic health. She leads the team 1 DO-IT "Diet and food matrix in Obesity: role of the Intestinal tract and innovative Therapeutics" of the CarMeN laboratory in Lyon (UMR Inserm U1060 / INRAE UMR1397 / University Lyon 1), labelled in March 2021 as a FRM team by the Foundation for Medical Research. Marie-Caroline Michalski conducts her work in close collaboration with the Rhône-Alpes Human Nutrition Research Centre within the CENS-ELI building dedicated to nutrition and health on the site of the Lyon Sud Hospitals (HCL). Finally, it is a founding member of the UMT ACTIA BALI (Bioavailability, Food, Lipids, Intestine), alongside ITERG and Bordeaux Science Agro.
During a meal, what happens to the fat in the food once it is ingested?
In the period following a meal, which we call the postprandial period, nutrients from the food we digest are absorbed in the gut. Their intestinal absorption, together with the metabolic events that occur during this postprandial period, play a major role in the development of metabolic and cardiovascular diseases. Concerning lipids more specifically, their postprandial metabolism is the subject of much research
It is known that an increase in the quantity of lipids ingested favours an increase in blood lipid parameters such as triglycerides (concentration of triglycerides in the blood). It has been shown that a prolonged increase in blood triglycerides for several hours after eating is associated with an increased cardiovascular risk. When our body's ability to use fat as an energy source is reduced in the context of a low energy requirement, the storage of unused fat is favoured in the adipose tissue. Once this storage capacity is exceeded, fat can then accumulate in other tissues (liver, heart). This phenomenon is characteristic of obesity and the metabolic syndrome.
To date, we can consider that the metabolic effects related to the quality and quantity of ingested lipids have been extensively studied, but it remains to be understood how their structuring in the food matrix or their formulation can also have consequences on health.
How are fats organised in our food?
In our diet, lipids can be present in different forms depending on the presence or absence of other constituents/nutrients and their level of formulation. In vegetable oils, lipids are in their simplest form, unformulated. Lipids can also be emulsified, i.e. associated with an aqueous phase in the form of oil-in-water droplets, an organisation that we find in salad dressings and mayonnaises, milk, or even yoghurts, which are gelled emulsions. Conversely, lipids can constitute a continuous phase in which water droplets are present; this is known as a water-in-oil emulsion: margarine and butter are examples. Finally, lipids can be present as inclusions in matrices combining carbohydrates or proteins, as in the case of biscuits and cheese.
Of these different levels of organisation, the emulsified form is the most common in the diet. Recent advances in nutritional research have shown that the type of structuring of fat, and in particular its emulsified structure, can have an impact on the digestion and bioavailability of the fatty acids that constitute dietary lipids.
What are the factors related to the formulation of lipids in food that can modulate their absorption?
The emulsification (formulation of lipids in emulsified form) of an oil modulates postprandial lipemia (quantity of lipids circulating in the blood plasma following a meal), as well as the use of dietary lipids by our body.
In collaboration with ITERG, we studied the impact of emulsification on intestinal lipid absorption. Using a plant-based emulsifier (lecithin) commonly used in food formulation, we have shown that the administration of an omega-3 rich oil (linseed oil in emulsified form) increases the intestinal absorption of the fatty acids contained in the oil, compared to the non-emulsified form [1][2]. The use of the same lecithin to emulsify flaxseed oil has also demonstrated better intestinal absorption and better secretion of omega 3 into the lymph compared to another emulsifier used in the food industry (sodium caseinate) [2].
Finally, our team has also shown in the context of work carried out on milk fat globules, which are naturally emulsified forms, a lipemic response of greater amplitude when milk fat droplets are coated with vegetable lecithin compared to caseins, proteins of dairy origin [3].
Do we have comparable data in humans?
Few data exist in humans. Some have shown that when healthy subjects consume an oil rich in polyunsaturated fatty acids, whether emulsified or not, the lipemia kinetics obtained for the two forms ingested are very different. The emulsified form thus increases the absorption of certain fatty acids such as omega 3 over a long period after the meal. It is in this context that my team proposed the concept of fast and slow lipids [4].
Although the studies published to date have demonstrated the impact of fat structuring on lipid absorption and the resulting lipaemia, they have not followed in detail the final metabolic fate of fatty acids after their intestinal absorption. In previous work, we have shown that different emulsified structures induce different kinetics of lipid utilisation as an energy source.
All these results led us to test the hypothesis that, beyond the kinetic aspects linked to a more or less rapid absorption speed, the use of dietary fatty acids by the body could be modified by the structure of the fat intake in the meal.
How can we limit or even prevent the metabolic disorders associated with obesity?
In the LIPINFLOX clinical trial we showed that fat in emulsified form, included in a realistic breakfast, induced a faster and higher peak in triglyceride levels compared to the same non-emulsified fat in spread form, thus showing a facilitated intestinal absorption [4]. This phenomenon is more important in obese men than in normo-weighted men and shows that the stimulating effect of emulsification on digestive lipolysis and absorption can occur in the context of a complex meal. The metabolic importance of such a change in the lipemia profile should not be overlooked. Indeed, as discussed above, sustained high postprandial lipemia is recognised as an independent risk factor for cardiovascular disease.
An innovative aspect of the LIPINFLOX study was to investigate the ultimate metabolic fate of ingested fatty acids by integrating isotopic tracers into the fat studied. Using a carbon-13 respiration test, we were able to demonstrate a higher utilisation of fatty acids ingested in emulsified form as an energy source. In the plasma of subjects who consumed the emulsified fat, we observed an influx of tracer fatty acids in a form that allowed their use as an energy source. This peak in dietary fatty acids can be explained by the better intestinal absorption of emulsified lipids and may explain their better and faster utilisation as an energy source.
What should we learn from all this?
In conclusion, although the diversity of lipid structures in foods is now better known, the effect of these structures on lipid digestion and the release of fatty acids in the digestive tract is being studied more and more. Research shows that the emulsified structure in particular can direct the metabolic fate of fatty acids in the body. This has led us to propose the concept of "fast lipids/slow lipids" for the modulation of the metabolic use of fatty acids by the structuring of the fat in the food.
The current high consumption of formulated products containing emulsified oils and fats and various surfactants (e.g. pastries and cakes, mixed dishes) reinforces the interest in knowing more about the metabolic effect of emulsified structures and emulsifying agents used in these products. Moreover, recent work by our team in collaboration with ITERG shows that not all natural emulsifiers seem to have similar metabolic effects [5][6]. This work includes taking into account the intestinal microbiota, which we are studying in the framework of the ACTIA Joint Technology Unit "Biodisponibilité Alimentation Lipides Intestin" (BALI), in collaboration with our Bordeaux partners (ITERG and the LMBA Laboratory of Bordeaux Sciences Agro) [7].
These elements therefore encourage further study of the effects of emulsifiers in humans, particularly in the context of the development of nutritional strategies capable of limiting or preventing the metabolic disorders associated with obesity [8][9].